The invention relates to bond coats. In particular, the invention relates to roughened bond coats for thermal barrier coating systems.
Thermal barrier coating systems are used in hot-section components in turbine and turbine components, for example components of jet engines and gas turbines. The thermal barrier coating system insulates the turbines from high temperatures during thermal cycling. Thermal barrier coating systems include a thermal barrier coating (TBC) disposed on a bond coat, which in turn is disposed on a substrate. The thermal barrier coating normally comprises zirconia, for example such as one of a stabilized zirconia and a partially-stabilized zirconia (PSZ). The bond coat typically comprises an oxidation-resistant metallic layer disposed between the TBC and substrate (turbine component). The TBC is adhered to the bond coat typically by mechanical interlocking, so the bond coat provides oxidation resistant to the substrate and a relatively rough surface. The bond coat surface generally has Ra (Arithmetic Average Roughness (Ra) as determined from ANSI/ASME Standard B461-1985) values over about 350 mainly by mechanical interlocking. So the function of the bond coat is to provide oxidation resistant to the substrate and a relatively rough surface, preferably with Ra values over about 350 microinches, for the TBC to adhere to the substrate. Thus, the TBC is disposed over the turbine component can provide thermal insulation.
FIG. 1 is a schematic representation of a known thermal barrier coating system 1. A substrate 10 comprises an underlying part of a component, for example a turbine component. A bond coat 12 is disposed on the substrate 10. The bond coat is disposed on the substrate 10 by any appropriate method, for example, but not limited to, thermal spray processes, such as vacuum plasma spray (VPS), air plasma spray (APS) and hyper-velocity oxy-fuel (HVOF) spray processes.
The structure and roughness of bond coat surface 13 are dependent on the spray process. Bond coats deposited by a VPS process are typically dense and free of oxides. Therefore, VPS-applied bond coats provide protection at high temperatures against oxidation. The VPS application process disposes fine powders, and thus, VPS-applied bond coats are typically dense, for example having a density greater than about 90% of its theoretical density, but have relatively smooth surfaces. Consequently, a TBC does not adhere well to a VPS bond coat.
An air plasma spray (APS) process produces rough bond coats because of large powders used in APS. The large powders possess a relatively high heat capacity; however, the APS-applied bond coats contain high amounts of oxides. Also, APS-applied bond coats possess a relatively low porosity due to the oxidation environment and low momentum of the powders. Although APS-applied bond coats provide better TBC adhesion due to their roughness, they are more prone to oxidation because of their relatively high oxide levels and relatively low porosity.
Bond coats deposited by HVOF are sensitive to particle size distributions. Dense and oxide-free bond coats can be deposited by HVOF using very lean conditions (low oxygen amounts) and finer particles, for example particles with a size about xe2x88x92325+10 xcexcm. The surface roughness of HVOF-applied bond coats is relatively smooth. Rough bond coats can be deposited by HVOF using coarser powders, for example particles with a size about xe2x88x92230+325, however a low HVOF flame temperature is needed. The low flame temperatures result in the bond coat comprising un-melted powders, therefore the coating is porous and less dense.
A TBC 14 is disposed on the bond coat 12 and forms a surface 15 against the surface 13. The TBC 14 is disposed on the bond coat 12 by any appropriate process to adhere (bond) to the bond coat. The TBC surface 15 and bond coat surface 13 define an interfacial area 16 at their adjoining surfaces.
Effectiveness of a thermal barrier coating system during thermal cycling is compromised by de-bonding of the TBC and bond coat, for example at the TBC and bond coat interfacial area. De-bonding can be caused by at least one of a poor TBC and bond coat adhesion, and lack of accommodation of thermal expansion mismatch between the TBC and bond coat. The lack of adhesion is characteristic of smooth adjoining surfaces where a total surface area is minimal. The thermal expansion mismatch between the TBC and bond coat results from different coefficients of thermal expansion of the materials used for these features. If the difference in coefficients of thermal expansion of the adhered elements is large, one element expands much more than the other, and separation and de-bonding occur at the interfacial areas. De-bonding of the TBC and bond coat is undesirable as the insulation effect of the thermal barrier coating system will be lost at TBC separation.
Therefore, it is desirable to use a very dense and rough bond coat that provides oxidation resistance and promotes enhanced adhesion between the TBC and the bond coat. The adhesion between the TBC and bond coat can be increased by increasing an area at an interfacial area mating surface of adhered elements. Increasing a roughness of the bond coat provides an increased area and enhanced mechanical interlocking between the bond coat and TBC. Increasing a bond coat""s roughness also provides an increased interfacial surface area for accommodation of any thermal mismatch, with respect to non-roughened bond coats.
Thus, this invention overcomes the above noted deficiencies of known bond coats and thermal barrier coating systems. The invention provides a method for providing a dense (for example at least about 95% its theoretical density), roughened bond coat, for example on a substrate, such as a turbine component, in a thermal barrier coating system. The method comprises providing an oxidation-resistant plasma-sprayed layer onto a substrate; and disposing a slurry overlayer on the oxidation-resistant plasma-sprayed layer to form a roughened bond coat possessing an uneven, undulated, and irregular surface.
A dense, (for example at least about 95% its theoretical density), roughened bond coat is also set forth in an embodiment of the invention. The roughened bond coat comprises an oxidation-resistant plasma-sprayed layer and a slurry overlayer on the oxidation-resistant plasma-sprayed layer to form an uneven, undulated, and irregular surface.
Further, a method for providing a thermal barrier coating system is disclosed in another embodiment of the invention. The thermal barrier coating system comprises a dense (for example at least about 95% its theoretical density), roughened bond coat and a thermal barrier coating disposed on a substrate. The method of providing a thermal barrier coating system comprises disposing a roughened bond coat on the substrate and disposing a thermal barrier coating on the roughened bond coat. The disposing a roughened bond coat comprises providing an oxidation-resistant plasma-sprayed layer onto a substrate and disposing a slurry overlayer on the oxidation-resistant plasma-sprayed layer to form an uneven, undulated, and irregular surface.
A thermal barrier coating system, as embodied by the invention, comprises a dense (for example at least about 95% its theoretical density), roughened bond coat disposed on a substrate and a thermal barrier coating disposed on the roughened bond coat. The roughened bond coat comprises an oxidation-resistant plasma-sprayed layer and a slurry overlayer on the oxidation-resistant plasma-sprayed layer. These layers form the roughened bond coat possessing an uneven, undulated, and irregular surface.
These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, where like parts are designated by like reference characters throughout the drawings, disclose embodiments of the invention.